frontend, api: allow disabling login methods individually

db/20190418210855-configurable-auth.sql

@@ -0,0 +1,16 @@
+-- Make all login providers optional (but enabled by default)
+
+ALTER TABLE domains
+  ADD commentoProvider BOOLEAN NOT NULL DEFAULT true;
+
+ALTER TABLE domains
+  ADD googleProvider BOOLEAN NOT NULL DEFAULT true;
+
+ALTER TABLE domains
+  ADD twitterProvider BOOLEAN NOT NULL DEFAULT true;
+
+ALTER TABLE domains
+  ADD githubProvider BOOLEAN NOT NULL DEFAULT true;
+
+ALTER TABLE domains
+  ADD gitlabProvider BOOLEAN NOT NULL DEFAULT true;